1. Field of the Invention
The present invention relates to a memory device including a semiconductor.
2. Description of the Related Art
There are many kinds of memory devices including semiconductors. Examples are a dynamic random access memory (DRAM), a static random access memory (SRAM), an electrically erasable programmable read only memory (EEPROM), a flash memory, and the like (see Patent Documents 1 and 2).
In a DRAM, data is stored by holding charge in a capacitor which is provided in a memory cell. However, even when a transistor used for switching is in an off state, a slight amount of leakage current is generated between a source and a drain; thus, the data is lost within a relatively short time. Therefore, the data needs to be rewritten (refreshed) on a regular cycle (generally once every several tens of milliseconds).
In an SRAM, data is held by utilizing a bistable state of a flip-flop circuit. A CMOS inverter is generally used in a flip-flop circuit of an SRAM; however, six transistors are used in one memory cell and the degree of integration of the SRAM is lower than that of a DRAM. In addition, the data is lost when power is not supplied.
On the other hand, in an EEPROM or a flash memory, a so-called floating gate is provided between a channel and a gate and charge is stored in the floating gate, whereby data is held. In this specification, a memory having a floating gate, such as an EEPROM or a flash memory, is called a floating-gate nonvolatile memory (FGNVM). The charge stored in the floating gate is held even after power supply to a transistor is stopped, which is why these memories are called nonvolatile memories.
Since multilevel data can be stored in one memory cell in an FGNVM, storage capacity can be large. Further, since the number of contact holes can be significantly decreased in a NAND flash memory, the degree of integration can be increased to some extent.
However, in an FGNVM, high voltage is needed at the time of injection of charge to a floating gate or removal of the charge. Because of this, deterioration of a gate insulating film cannot be avoided and it is not possible to limitlessly repeat write and erase operations.